Surface-emitting laser diodes (known as Vertical-Cavity Surface-Emitting Lasers: VCSEL) are increasingly used in modern optoelectronic systems, on account of their numerous advantages such as low threshold currents and symmetrical lobes and are gradually replacing the conventional edge-emitting semiconductor lasers. Because of their more or less distinct transverse symmetry (rotational symmetry) the VCSELs have no or only inadequate polarization selectivity. In use, this may lead to polarization instabilities and polarization switches, ruling out the use of such lasers for most applications.
Surface-emitting laser diodes generally have a cylindrically symmetrical structure and on the basis of their design and manufacturing method have no preferential direction for the direction of polarization of the radiated wave. There are therefore two orthogonal states relating to the direction of polarization of the radiated wave. In an ideal laser structure, these two states are energetically degenerate and are equally suitable for laser operation. However, because of the electro-optical effect and anisotropies in the component design as well as asymmetries and fluctuations in the manufacturing process, this degeneracy is eliminated and the VCSEL oscillates dominantly only in the preferred polarization mode. In most cases the mechanism that leads to a particular mode being preferred is difficult to control or not obvious and poorly distinguished, with the result that the polarization process as a whole is random and unstable in nature. The polarization switches generally limit use in polarization-dependent optical systems. For example, such switches in optical data transmission lead to increased noise. As numerous applications rely on polarization-stable lasers as light sources, this represents a significant reduction in the production yield. In many cases a preferential direction can indeed be defined but the elimination of the degeneracy is not powerful enough to guarantee polarization stability under varying environmental and operational conditions. In this case, even minor changes in these parameters may cause alternation between the two states (“pole flip”).
In the past, numerous possible solutions for stabilizing polarization were studied. In order to achieve polarization stability for GaAs-based VCSEL, growth was successfully shown on higher-indexed [311] substrates in O. Tadanaga, K. Tateno, H. Uenohara, T. Kagawa, and C. Amano, “An 850-nm InAlGaAs Strained Quantum-Well Vertical-Cavity Surface-Emitting Laser Grown on GaAs (311)B Substrate with High-Polarization Stability”, IEEE Photon. Technol. Lett., 12, 942 (2000). However, since the other laser properties generally deteriorate and there are difficult growth conditions particularly for InP-based semiconductor layers, this method does not appear to be suitable for long-wave VCSEL.
Another approach to the problem is the use of dielectric or metallic grating structures as in J.-H. Ser et al., “Polarization stabilization of vertical-cavity top-surface-emitting lasers by inscription of fine metal-interlaced gratings”, Appl. Phys. Lett. 66, 2769 (1995); T. Mukaihara et al., “Polarization control of vertical-cavity surface-emitting lasers using a birefringent metal/dielectric polarizer loaded on top distributed Bragg Reflector”, IEEE J. Sel. Top. Quantum. Electron. 1, 667 (1995); M. Ortsiefer et al., “Polarization Control in Buried Tunnel Junction VCSELs Using a Birefringent Semiconductor/Dielectric Subwavelength Grating”, IEEE Photon. Technol. Lett., 22, 15 (2010) and P. Debernardi et al.: “Reliable Polarization Control of VCSELs Through Monolithically Integrated Surface Gratings: A Comparative Theoretical and Experimental Study”, IEEE J. Sel. Top. Quantum Electron. 11, 107 (2005). Metallic-dielectric gratings with subwavelength dimensions are used to produce birefringence in the laser resonator. Thus the optical resonator length, or the resonating frequency of the laser resonator, can only correspond to the maximum of the Bragg reflector or reflectors during one polarization. The other polarization is thus suppressed. The dielectric gratings proposed, on the other hand, make use of interference effects in the grating, as a result of which the total reflection is amplified or attenuated by the grating, dependent on polarization. The periodicity of the corresponding grating structures should not therefore be below approximately half a vacuum wavelength. One approach for the BTJ (Buried Tunnel Junction)—VCSEL produced by the Applicant with integrated subwavelength gratings of short periodicity (<λ/2) was recently proposed by M. Ortsiefer et al., “Polarization Control in Buried Tunnel Junction VCSELs Using a Birefringent Semiconductor/Dielectric Subwavelength Grating”, IEEE Photon. Technol. Lett., 22, 15 (2010). Generally, the technology of the subwavelength gratings is complicated and laborious (nanostructuring), particularly when they have to be filled with other dielectrics, even though the advantages are clear.
JP 2005-093858 A describes a VCSEL with a birefringent layer on the outside of a mirror of the resonator. This leads to a preferential direction of polarization, however, to a reduction in the total reflection.
In US 2008/112443 A1 is shown a VCSEL with an external resonator mirror. Inside the resonator is arranged a polarization-selective layer.
WO 2007/057807 A2 of Philips Forschungslaboratorien proposed, in 2005, inserting a layer with a polarization-dependent refractive index or polarization-dependent absorption within the VCSEL resonator in front of one of the two end mirrors, so that the effective mirror reflection becomes polarization-dependent and thus one polarization mode is preferred and is actuated. A serious disadvantage of this method, and of all processes that place polarization-selective elements inside the laser resonator, is the simultaneous effect on the (optical) resonator length and hence on the laser wavelength. There may also be losses in the anisotropic layers, particularly in layers applied by oblique angle vapor deposition, which could noticeably impair the laser properties (e.g. the threshold). Moreover, a more complicated manufacturing technology and a reduced yield are obtained when the laser wavelength has to be adhered to precisely.